The cleverer managers I've worked with typically include a factor of ~ 70-80% on worker office time. So in a typical 8 hour work day, 6 of those hours would be productively spent on a task at hand.

For an engineer who is sitting at his or her desk, focussed on a single task, I think this is reasonably accurate. Obviously there are a lot of things in an office environment which can further conspire to reduce the amount of productivity much, much lower.